1. Field of the Invention
The present invention relates to a method for producing semiconductor device using a crystalline semiconductor.
2. Description of the Related Art
Thin film transistors (TFTs) using thin film semiconductors are known. These TFTs are constructed by using a semiconductor thin film on a substrate thereof. These TFTs are used in various integrated circuits. They are noted as a switching element particularly in an electro-optical device, and more particularly as a switching element provided in each pixel in an active matrix type liquid crystal display (LCD) device, and as a driver element formed in a peripheral circuit portion.
With semiconductors thin film used in the TFTs, an amorphous silicon film can be conveniently used. However, such silicon film has a problem in that the electric characteristics are low. To improve the characteristics of the TFTs, crystalline silicon thin films may be used. Crystalline silicon films are referred to as a polycrystalline silicon, polysilicon, fine crystal silicon or the like. To obtain such a crystalline silicon film, an amorphous silicon film may be formed first followed by heating the film to crystallize it.
However, there is a problem in that crystallization of the silicon film by heating requires a heating temperature of 600.degree. C. or more and a heating time of 20 hours or more, a glass substrate can be used with great difficulty. For example, Corning 7059 glass has a glass skew point of 593.degree. C. In consideration of the maximization of the substrate, heating the silicon film at 600.degree. C. is problematic. In other words, when Corning 7059 glass substrate often used is heated at 600.degree. C. or more for 20 hours or more, shrinkage and warp of the substrate become very conspicuous.
To solve the above problem, heating at the lowest possible temperature is required. On the other hand, heating process time is demanded to be shortened as much as possible to heighten the productivity.
When the amorphous silicon film is crystallized by heating, there arises a problem in that the whole silicon film is crystallized, which impedes partial crystallization of the substrate or a control of crystallization of a specific region of the substrate.
As a method for solving the above problem, Japanese Unexamined Patent Application No. HEI 2-140915 and Japanese Unexamined Patent Application No. HEI 2-260524 describe a technique for selectively crystallizing the substrate by artificially forming a portion or a region that will constitute a crystal nuclei in the amorphous silicon film followed by heating the substrate.
Japanese Unexamined Patent Application No. HEI 2-140915 describes a structure of crystal growth from this crystal nuclei by forming an aluminum layer on the amorphous silicon film and growing the crystal nuclei in a portion where this amorphous silicon contacts the aluminum followed by heating the silicon substrate. Further, Japanese Unexamined Patent Application No. HEI 2-260524 describes a structure for doping tin (Sn) into the amorphous silicon by the ion doping method to grow a crystal nuclei in a region into which tin ions are implanted.
However, Al and Sn are substitute metals which constitute an alloy with silicon so as not to diffuse into and intrude the silicon film. Then the portion of the metal which forms an alloy core together with silicon constitutes a crystal nuclei. The crystal is allowed to grow from that portion. When Al and Sn are introduced, the crystallization is characterized in that the crystal growth proceeds from the portion at which Al and Sn are introduced (an alloy layer of Al and Sn, and silicon). Generally, the crystallization proceeds in two stages; generation of an initial core and the crystal growth from the core. Al and Sn which are substituent metal elements with respect to silicon are effective in the generation of the initial core, but is hardly effective in the crystal growth after that.
Consequently, when Al and Sn are used, the temperature cannot be set to a low level compared with the crystallization of the amorphous silicon film by heating nor the heating time can be set to a short level. In other words, use of Al and Sn shows no remarkable superiority over the crystallization process of amorphous silicon film which is carried out by the conventional simple heating.